1. Field of the Invention
This invention relates generally to ocean bottom or seabed seismic surveying, and, more particularly, to cable motion detection in ocean bottom or seabed seismic surveying.
2. Description of the Related Art
Underwater seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying one or more and one or more seismic sensors at predetermined locations. For example, a seismic cable including an array of seismic sensors may be deployed on the sea floor and a seismic source may be towed along the ocean's surface by a survey vessel. The seismic sources generate acoustic waves that travel to the geological formations, where they are reflected and propagate back to the seismic sensors. The seismic sensors receive the reflected waves, which are then processed to generate seismic data. Analysis of the seismic data may indicate probable locations of geological formations and hydrocarbon deposits.
The accuracy of the seismic survey depends, at least in part, on an accurate determination of the location of the deployed cable during the survey, so the position of the deployed seismic cable is typically verified before conducting the seismic survey. For example, following the deployment of the cable, an initial position of the deployed cable is determined using an initial position determination operation. The initial position determination operation may be performed using a variety of well-known techniques. For example, one or more survey vessels may make direct measurements of the location of transceivers attached to the cable during passes along either side of the cable.
The deployed cable often moves during the seismic survey. For example, high currents and/or turbulence in the water may shift the position of the cable. If the seismic cable moves more than an acceptable distance from its initial position, it may be necessary to correct the seismic survey data, e.g., during subsequent data processing. Thus, despite the high cost of performing position determination operations, a final position of the cable may be verified using a final position determination operation after the seismic survey is complete. For example, the probability that the seismic cable has moved more than an acceptable distance from its initial position may be increased if the weather has been particularly bad during the course of the seismic survey, or the seas have been particularly high, and performing the final position determination operation may then be considered a worthwhile investment of time and resources.
However, the results of the initial and final position determination operations may incorrectly indicate that the cable has moved. For example, variations in the speed of sound in the water near the seismic cable may alter the propagation velocities of the acoustic waves used in the position determination operations, which may result in different determined positions of the seismic cable. For another example, these so-called “misclosures” between the initial and final position determinations may be caused by variations in the inclination angle between the survey vessel and the cable.
A variety of techniques are traditionally used to determine whether cable motion is the cause of the misclosures. In one conventional method, noise from the seismic sensors is monitored so that noise produced by motion of the seismic sensors may be detected. However, distinguishing motion-induced noise from other types of noise that may also be produced by the seismic sensor requires using complex logic devices that may not always accurately distinguish between the various types of noise. In another conventional method, the misclosures caused by acoustic propagation variations are modeled and compared to the measured misclosures. However, fitting the misclosures caused by acoustic propagation variations to an accurate model may be difficult, time-consuming, and frequently unsuccessful.
Moreover, even if it can be determined that the misclosures were caused by motion of the cable, it is very difficult, and often impossible, to determine when the movement of the cable occurred. Consequently, the location of the cable during the seismic survey may remain uncertain, which may reduce the accuracy of the results of the seismic survey.